In a continuous type (as distinguished from drop on demand type) ink jet printer, conductive ink is forced through an orifice to form an ink filament. The ink is vibrated at a constant frequency to cause drops to regularly separate from the end of the filament. A drop charging electrode located in the vicinity of the drop separation point is employed to selectively induce an electric charge on the conductive ink filament at the instant of drop separation. The separating drop captures the electric charge induced on the filament, and the charged drop is electrostatically deflected, for example to a drop catcher.
In a continuous binary type ink jet printer, all charged drops are deflected along a catch trajectory to a drop catcher, and uncharged drops proceed along a print trajectory to a print receiving surface such as paper.
In another type of ink jet printer, various amounts of charge are selectively placed on the drops to deflect the drops along respective print trajectories, or along a catch trajectory.
The print head of a prior art continuous ink jet printer generally includes a reservoir for delivering the conductive ink, under pressure, to an ink jet orifice; means for vibrating the ink, such as a piezoelectric transducer attached to the reservoir; a charging electrode for selectively charging the ink drops as they separate from the filament; means for deflecting the charged drops; and means for catching nonprinting drops.
In many prior art ink jet printing heads for continuous ink jet printing, the charging electrode is a small cylindrical conductor, generally formed by plating the inside of a microscopic hole in an insulating sheet of material. The means provided for deflecting the charged ink drops generally comprises a pair of deflection electrodes, arranged like the plates of an air capacitor, to which a constant deflection voltage is applied to establish an electrostatic deflection field between the plates. The ink drops travel between the plates, and the charged drops are deflected by the electric field.
U.S. Pat. No. 3,656,171, issued Apr. 11, 1972 to J. A. Robertson, discloses an ink jet printing head for continuous ink jet printing, wherein the drop charging electrode is simply a plate disposed adjacent the ink jet filament in the vicinity of drop separation, and the means for deflecting the ink drops is simplified to an electrically conductive surface that is arranged along one side of the path of the ink drops. The construction of such an ink jet printing head was substantially simpler than the prior art ink jet print heads.
The theory proposed by Robertson regarding the operation of the deflection means was that the charged ink drops induced a mirror charge in the conductive surface. The charged drops were then attracted to their mirror images, thereby causing the deflection. This arrangement had the advantage of simplifying the construction of the ink jet printing head by eliminating one half of the previously required deflection electrode structure, and eliminating the need for a separate high voltage power supply to charge the deflection electrodes in the ink jet printer apparatus.
It was further noted by Robertson that the deflection means may comprise an extension of the charging electrode, thereby further simplifying the construction of the ink jet print head.
Such an arrangement is shown in FIG. 3, where an ink jet printing head 10 includes an ink reservoir 12 containing conductive ink 14 under pressure. The ink is forced from an an orifice 16 to form an ink filament 18. The ink is vibrated by means not shown, to cause ink drops 20 to regularly separate from the ink filament 18. The ink drops 20 are selectively charged by a voltage V.sub.C applied to an extended drop charging electrode 22 from a charging circuit 24. Charged drops are deflected into a drop catcher 26, and uncharged drops proceed to the printing surface 28.
Although this approach has the advantage of eliminating all together the need for a separate deflection electrode, the extended charging electrode protrudes into the region where deflected drops may impact on the vertical face of the drop catcher. This is undesirable, since the conductive ink stream may then ground the charging electrode to the ink supply, or in the case of a multi-jet print head, short adjacent charging electrodes. It is the object of the present invention therefore to provide a simplified ink jet printing head of the type having a simple planar drop charging electrode located adjacent an ink jet filament in the region of the drop separation, that is free from the shortcomings noted above.